Conventionally, some diesel engines have selective reduction catalyst incorporated in an exhaust pipe through which exhaust gas flows, said catalyst having a feature of selectively reacting NOx with a reducing agent even in the presence of oxygen. A required amount of reducing agent is added upstream of the reduction catalyst and is reacted on the catalyst with NOx (nitrogen oxides) in the exhaust gas to thereby reduce a concentration of the discharged NOx.
In a field of industrial plant or the like with flue-gas denitration, it has been well known that ammonia (NH3) is effectively used as reducing agent to reduce and depurate NOx through reduction. However, for automobiles, safety in carrying ammonia itself during running is difficult to ensure, so that in recent years, use of nontoxic urea water as reducing agent has been researched.
More specifically, when urea water is added to the exhaust gas upstream of the selective reduction catalyst, the urea water is decomposed into ammonia and carbon dioxide gas according to the following equation to thereby depurate NOx in the exhaust gas well through reduction by ammonia on the catalyst.(NH2)2CO+H2O→2NH3+CO2 
For exhaust emission control of the diesel engine, mere removal of NOx in the exhaust gas is insufficient; particulates (particulate matter) in the exhaust gas must be captured through a particulate filter. This kind of particulate filter employed requires to be timely regenerated by burning off the particulates to prevent increase of exhaust resistance due to clogging.
To this end, it has been conceived to additionally arrange flow-through type oxidation catalyst in front of the particulate filter; with accumulation of the particulates becoming increased, fuel is added to the exhaust gas upstream of the oxidation catalyst to forcibly regenerate the particulate filter.
More specifically, when the fuel is added to the exhaust gas upstream of the oxidation catalyst, the added fuel (HC) brings about oxidation reaction during its passing through the oxidation catalyst, so that inflow of the exhaust gas elevated in temperature by the reaction heat of the oxidization elevates a temperature of a catalytic floor of the particulate filter just behind to burn off the particulates, thereby attaining regeneration of the particulate filter.
As actual measures for carrying out the above-mentioned fuel addition, it has been generally conceived that main injection of the fuel near a compression upper dead center is followed by post injection at non-ignition timing after the compression upper dead center so as to add the fuel to the exhaust gas. For effective utilization of the added fuel in forced regeneration of the catalyst and in order to conduct oxidization treatment of the added fuel before substantial lowering in temperature of the exhaust gas, it has been conceived preferable to arrange the particulate filter upstream of the selective reduction catalyst (see, for example, the following Patent Literature 1).    [Patent Literature 1] JP 2005-42687A